Well, there was a small minority in the Ionian Island in 1800 when the English came but I guess that after the union with Greece many were assimilated(there was even an island with Italian or Venetian majority).
Anyway saying that Italy has no claims in these island is quite extreme, there were even if not stronger as the the Greek one.
I don´t think Crete had a Italian minority so here is more like a colonial expansion than "retake our land".
The island you mention is Zante (or Zakhyntos in Greek). There was obviously a significant number of Italians (or better Venetians) in the Ionic islands up to the end of the Serenissima, but they were never more than 10-15% of the total population and started to decrease during the Napoleonic period. The remaining population left the islands after the union to Greece (1864) and migrated either to Italy or to Smyrna (under the Ottomans they were protected by the regime of capitulations, while in Greece the capitulations had been abolished). The first constitution of the Ionic islands (1803) was actually bilingual (Italian and Greek).
IMHO, Italy might have an historical claim to the islands but the practical disappearance of the Italian minority made the point moot. Certainly these islands were never included in the "Irredenta" package, and IMHO it was not a wise claim to pursue.
I thought they never had a majority, something like 33% in 1815 when Austria annexed it.
It is more a matter of how one defines the borders of Dalmatia, I suppose. The Italian-speaking populations was concentrated in the coastal cities, while Slavic population was predominant in the interior. Culture, commerce and professions were certainly dominated by Italian speakers. All the higher schools were Italian ones. The first newspaper in Dalmatia was bi-lingual, but it actually meant that the articles were written in Italian and then translated in Serbo-croat. Until late 1860s, all the mayors were Italian speaking in the cities (and in Spalato the last Italian-speaking mayor, Baiamonti, was elected without interruption from 1860 to 1880 (when the crown dissolved the city council and "arranged" for new elections two years later).
Interestingly enough, the Serbo-croat spoken by Slavic inhabitants in Dalmatia included a very large number of Italian (or Venetian) words: practically everything that had to do with sea-faring, commerce, culture, professions, fishing and so on (which was not the case for the Serbo-croat spoken around Zaghreb). Anyway most of the Slavic speakers were also speaking Venetian dialect and/or Italian.
There is also the difficulty of correctly estimating the number of persons belonging to each group? It was somehow easier to identify the people by religion since the churches kept records; however both Italians and Croats were catholic, and mixed marriages were common (it was again a matter of religion and social status, not an ethnic one).
Under the Republic of Venice this was not an issue. Anyway people were mostly feeling a sense of belonging to a church or to a town, not to an ethnic group. The first stirrings of nationalism came later, and this was a problem for the Austrians. Slavic people were considered loyal, Italians (in particular starting with the 1840) were not. The imperial government incentivated Slavic migration to Dalmatia and after the war of 1866 made it an official policy:
"His Majesty has expressed the precise order to oppose decisively influence of the Italian element still present in some Kronländer, and aim to Germanization or Slavicization - depending on the circumstances - the areas in question with all the energy and without any respect, through appropriate mandates of judges to politicians, educators and through the influence of the press in southern Tyrol , Dalmatia and the Adriatic Coast . » [quoted from Wikipedia]. Since the Catholic church was not friendly to the newly created kingdom of Italy (and census taking was in church) it is not surprising that the official number of Italian speakers (in the official definition of the KuK, which was the day-to-day language spoken at home) decreased.
Is it a good enough answer?